It is known from prior art various configurations and types of sensors based on the strain applied to an optical fiber. Amongst such types of sensors, the most popular physical properties that are measured are temperature, pressure and acceleration and the most popular type of strain-based sensors are FBG (Fiber Bragg Grating) based sensors.
Optical sensing is an increasingly used technology given its non-dependence on electrical signals and the possibility of including several types of sensors in a single fiber. Also, for harsh environments, such as oil wells, fiber optics provides advantages of high temperature and pressure operation range, low electromagnetic interference pick-up, high signal to noise characteristics and high number of sensors communicating with minimum size and number of cables.
In case of strain-based fiber optic sensing, one of the conditions that have to be considered while designing a sensor is the fact the strain is limited to a several μm and that, once those few μm are exceeded is relatively easy to break the fiber, which implies that the whole optical fiber has to be replaced and re-installed. This is especially challenging in accelerometers (or motion detectors) whereby the need for high precision often results in relatively large moving parts whose miniscule motion is transmitted to form a force stretching of the sensitive region of the optical fiber. In such sensors, shocks and large accelerations result in potentially destructive forces on the joints in the transmission mechanisms, on the (fiber) connection points and the fiber itself.
Therefore, in prior art there have been several mechanisms designed to avoid this over-straining of the fiber, one of such mechanisms involves the use of stoppers to prevent the fiber from overstretching, however, this stoppers require a very precise positioning down to few μm even sub-μm level which is hard to achieve in component manufacturing and assembly, resulting in relatively expensive systems. Furthermore, the manufactured gaps and tolerances for such stoppers will then have to remain stable even under very high pressure values, requiring very rigid housing designs.
Another solution known from prior art is to increase the hardness of the casings of the sensors by using harder materials for their construction or increasing their mass. However, this solution is no ideal because it will increase the overall mass of the sensor, making it harder to use in some environments. This solution also has the disadvantage that it only solves the casing damage issue and would not solve the problem of damage of the internal parts of the sensor.
Under some environmental conditions such as, for example, down-hole operations in the oil and gas industry, it is required that a sensor is capable of withstanding shocks with very high magnitudes from 100-1500 g-force * mass (14715*m [N], being m the mass of the sensor expressed in kilograms) while they are being installed i.e., before their operation starts. Also, such sensors must withstand, during operation, pressures of around 100-2100 bar (10-210 MPa) and temperatures up to around 300 degrees Celsius.
The submicron machining precision required is hardly achievable for the use of stoppers and, under forces of 1500 g-force * mass it may not be enough to maintain the integrity of other moving parts within the sensors. Also, increasing the hardness and the mass of the sensor to withstand such conditions would lead to sensors being too big to be used. Therefore, it is concluded that prior art sensors cannot meet the requirements of the challenging conditions of down-hole operations.
In most cases, however, the main issues on system failure due to high shocks is often during handling, transportation, preparations in the field and installation of the equipment. During this time, it is often the case that the equipment remains at temperatures well below its operation temperature. As such, it is essentially that the shock protection is at its best during the relatively low temperatures whereas the moving parts of the sensor are free to function with minimal resistance at the elevated operation temperatures.